The present invention is directed to devices and methods for generating light with electrodeless high intensity discharge (HID) lamps. More particularly, the present invention provides high intensity discharge lamps driven by a radio-frequency source without the use of electrodes inside a gas-filled vessel (bulb) and related methods. Merely by way of example, such electrodeless HID lamps can be applied to applications such as parking lots, street lights, warehouses, stadiums, security, ports and harbors, large and small buildings, vehicle headlamps, billboard lighting, building facade lighting, airports, bridges, agriculture and horticulture lighting, architectural lighting, stage and entertainment lighting, medical illumination, microscopes, projectors and displays, ultraviolet (UV) water treatment, UV curing, any combination of these, and the like.
High intensity discharge lamps provide extremely bright and broad spectrum light source. Typical conventional electroded HID manufactured today contains a bulb with a mixture of gas and metal halides that are excited to form a plasma using a high current passed through closely-spaced electrodes. This arrangement, however, suffers from deterioration of the electrodes over time, and therefore a bulb with continual degradation of performance and limited lifetime.
Electrodeless high intensity discharge lamps driven by radio frequency (RF) sources have been proposed in the prior art. Some configurations include a metal halide fill encased either in a bulb or a sealed recess within a dielectric body forming a waveguide, with RF energy being provided by an external source such as a magnetron or solid-state RF driver and introduced into the waveguide and heating the plasma resistively. Another example is provided by U.S. Pat. No. 6,737,809 B2, which shows a different arrangement. This patent shows an electrodeless bulb and a dielectric cavity forming a part of a resonant RF circuit with an RF driver (which produces and amplifies electromagnetic energy at radio frequencies) to provide excitation. Several limitations, however, exist with this approach. The dielectric cavity is spatially positioned around a periphery of the electrodeless bulb in an integrated configuration, which physically blocks a substantial portion of the light emitted from the bulb. In addition, the integrated ceramic and quartz bulb configuration is difficult to manufacture and limits the operation and reliability of the plasma-enclosing bulb. Furthermore, the dielectric material used in this approach is often costly because of the difficulty in manufacturing and the high purity that is required to achieve competitive performance versus other lighting technologies for most applications.
In another approach disclosed in U.S. Pat. Nos. 8,283,866 and 8,294,368, an air-cavity resonator with grounded coupling elements is used to provide advantages over previous dielectric waveguide/resonator approaches. The air cavity resonator eliminates the need to use costly dielectric material and the bulb is not surrounded by a dielectric material resulting in more efficient operation of the lamp. However, this approach has its own limitations. These lamps are typically operated at frequency of around 435 MHz which limits the conversion efficiency (direct current (DC) to RF conversion efficiency of ˜80%) of the solid-state RF driver. If one also combines the efficiency of the alternating current (AC) to DC power supply (which must be used when the lamp is connected to standard electrical sources provided by electrical utilities) the overall efficiency of the lamp's source (AC to RF conversion efficiency of <74%) can drop to levels less competitive against other alternative lighting technologies such as light-emitting diodes (LEDs), HIDs, and fluorescent lamps.
To improve the overall system efficiency of the light source to levels that are more competitive against alternative lighting technologies, the efficiency of the RF driver can be increased by operating at a much lower frequency than 400 MHz (for example, 100 MHz). By using an RF driver at a lower frequency, such as 100 MHz, it is possible to achieve RF driver conversion efficiency (DC-to-RF) exceeding 90% and overall conversion efficiency (AC-to-RF) of around 83% to 88%. This is a significant improvement over current approaches and can improve the overall system efficiency of the light source to match or exceed the efficiency of other alternative lighting technologies such as traditional HIDs and LEDs. In addition, increasing the efficiency of the RF driver from 80% to 90% results in significant reduction of the dissipated power by the RF driver providing important benefits such as reducing the size and cost of the required heat sink and improving reliability. Operation at lower frequencies, however, is complicated by the fact that prior embodiments of the RF waveguide/resonator and air-cavity resonators are limited and can only be operated at these lower frequencies with techniques that force the lamp to be prohibitively large in size, cost, and complexity, and can result in significantly lower light output and efficacy.
From above, it is seen that techniques for improved electrodeless high intensity discharge lighting are highly desired.